Well, I took Tae Kwon Do and an offspring of some crap called Choi Kwan Do for a total of four years. Got to Red belt in both, although I could have had a black belt in the Tae Knon Do, but I got tired of the long forums that had nothing to do with any real techniques. I only would show up for sparring because they had contact sparring, but no contact headshots with the hands. So yeah, you could kick to the head, but not punch. BS. I kind of got kicked out of there because I would grab anyone's leg and twist them to the ground if they tried to kick me in the head, or I would just block and punch to the face. The main instructor would not make a deal of it, he was pretty realistic, but some of the others got so pissed they wanted me to leave. The final straw was when I knocked their little protege cold. He tried to teach me a lesson by spin kicking me in the head hard, and he got me, but I came back with three hard hooks to his jaw and put him out. I had a bad headach and saw stars and felt dizzy, but I'm sure his was much worse. He had been studying for 14 years and lived for that place, and I took him out in about four seconds, and I'm no badass.
Anyways, what I figured is this, 99% of these martial arts places are just family places, more interested in the Art, they aren't really into really teaching anything you can really use.
Size and physical fitness? I kind of dissagree with that as being a main factor, although it is a factor. I learned in the USMC that intensity has so much to do with it. I know that you don't fight the other person's game. A short person is stupid to try to go toe to toe with a tall person, he needs to get on the inside and come up, kind of how Mike Tyson did. He would come in low, then up, worked very well. It would be stupid for a tall person to try to grapple out a short stocky person, he needs to stand back and keep the distance. Knowing how to take a hit, and give a hit is quite important. Just because you are strong doesn't mean you can hit real hard. You have to be able to generate power